1. Field of the Invention
The present invention relates to a Pulse Width Modulation (PWM) signal generation circuit and a PWM control circuit.
2. Description of the Related Art
Each country has its own safety standards, for example, for headlamps of vehicles. According to some safety standards, headlamps are required to be illuminated while driving in broad daylight at a brightness of a predetermined proportion (for example, 12.5% and 22.5%) to that of the nighttime. Therefore, some headlamps are conventionally illuminated with PWM control for headlighting during daylight (Daylights) in a predetermined proportion to headlighting at nighttime. Some arrangements for the PWM control accomplish the lighting control of headlamps based on PWM signals, for example, generated by a microcomputer or a special purpose IC.
Those arrangements for the PWM control with the microcomputer and the like, however, require the voltage of a battery mounted on the vehicle (for example, 12 V) to be converted to a voltage on the order of 5 V before supplying power to the microcomputer, resulting in the need of an arrangement to accomplish this end. In addition, because the reflectance of a reflector surrounding a headlamp is different according to the types, etc., of vehicles on which the headlamp is mounted, headlamps should be arranged so that the duty ratio of the PWM control is adjusted depending on the difference in the reflectance of the reflectors in order to meet the specific safety standard of a country. A PWM control with a microcomputer and the like, however, is programmed to output PWM signals for a predetermined duty ratio. The program itself must be modified in order to change the duty ratio. This poses a problem in that it is labor-intensive and time-consuming to change the duty ratio.
Meanwhile, for example, Japanese Laid-Open Patent No. 7-154965 discloses a PWM control circuit for PWM control with a hardware configuration including an oscillation circuit.
However, the PWM control circuit in Japanese Laid-Open Patent No. 7-154965 has a problem in that, as the circuit constant of a PWM control circuit varies due to changes in the ambient temperature, degradation of circuit elements, and the like, the duty ratio varies accordingly. Such a problem may have a significant impact, particularly on a vehicle etc., which is often used in an environment where the ambient temperature changes drastically.
The present invention has been made in view of the foregoing circumstances. It is an object of the invention to provide a PWM signal generation circuit and a PWM control circuit in which the duty ratio is easily changed and which can avoid a significant adverse impact from ambient temperature changes and the like.